The use of depressions into the surface of a semiconductor substrate to increase, for example, the capacitance of capacitor structures or the packing density of other circuit structures is quickly developing. For example, U.S. Pat. No. 4,651,184 issued Mar. 17, 1987 to Malhi shows an entire dynamic random access memory cell in a single high aspect ratio depression. Such depressions are commonly known as trenches.
Although the uses of trench techniques and the fabrication of integrated circuits are still being developed, some aspects of the trenches used which enhance the manufactureability and reliability of the integrated circuits using them are becoming clear. One aspect of great importance is that the trench profile must be carefully controlled, having positively sloping sidewalls, for example. In some prior art techniques, among other deleterious effects, negatively sloped sidewalls or bowing occurs, This leads to voids in the trench when refilled by subsequent fabrication processes and to shorts caused by polycrystalline silicon or metals filling the voids during subsequent manufacturing processes.
Another aspect is that the trenches should be capable of being etched as deeply as possible. Present techniques are limited by various problems in the process. For example, in copending application Ser. No. 730,701, now U.S. Pat. No. 4,702,795 issued Oct. 27, 1987, assigned to the present assignee, the depth of the trenches using the process described therein can be limited by several aspects: (1) Because the technique involves etching the mask to provide forward sputtering, the etch mask will at some point be entirely consumed and the etching must stop, (2) the forward sputtering process is not easily controlled and an overly positive slope of the trench sidewalls may result: the sloping sidewalls will at some point taper to a point where no more etching occurs, (3) the sidewall residue can accumulate to the point where the trench mouth is filled in, and (4) the sidewall angle of the mask materials influence the forward sputtering, and control of the mask sidewall is not easily achieved.